User manual

Small Vision System User Manual 29
2.5 Filtering
Stereo processing will generally contain incorrect matches. There are two major sources for these
errors: lack of sufficient image texture for a good match, and ambiguity in matching when the correlation
window straddles a depth boundary in the image. The SVS stereo processing has two filters to identify
these mismatches: a confidence measure for textureless areas, and a left/right check for depth boundaries.
Areas that are filtered appear black in the displayed disparity image. To distinguish them from valid
disparity values, they have the special values 0xFFFF (confidence rejection) and 0xFFFE (left/right
rejection).
2.5.1 Confidence Filter
The confidence filter eliminates stereo matches that have a low probability of success because of lack
of image texture. There is a threshold, the confidence threshold, that acts as a cutoff. Weak textures give a
confidence measure below the threshold, and are eliminated by the filter.
The confidence threshold is adjusted using the Conf spin control in the Parameters area. A good
value can be found by pointing the stereo cameras at a textureless surface such as a blank wall, and starting
the stereo process. There will be a lot of noise in the disparity display if the confidence threshold is set to
0. Adjust the threshold until the noise just disappears, and is replaced by a black area.
The computational cost of the confidence filter is negligible, and it is usually active in a stereo
application.
2.5.2 Left/Right Filter
Each stereo camera has a slightly different view of the scene, and at the boundaries of an object there
will be an area that can be viewed by one camera but not the other. Such occluded areas cause problems
for stereo matches. Fortunately, they can be detected by a consistency check in which matching is done
first by using the left image as a fixed base, and then repeating the match using the right image as the base.
Disparity values for the same point that are not the same fail the left/right check. Typically, this will occur
near the boundaries of objects.
The left/right check is controlled by three radio buttons in the Parameter area. It can be turned on
or off. A third option is to perform the check, but instead of discarding disparity values that are
inconsistent, use the one that is smaller (further away). This option can fill in the areas around object
borders in a reasonable way. It is not currently available under MMX processing.
The left/right check adds about 20% to the computational cost of the stereo process, but is usually
worth the effort.